Sodium metal, which is a representative example of readily oxidizable and hygroscopic materials, reacts with oxygen and moisture in the air to produce sodium oxide and sodium hydroxide. In a process of growing nitride crystals using sodium metal as a flux component, the produced sodium oxide and sodium hydroxide are responsible for inhibiting nitridization, and thus it has been necessary that oxygen and moisture be prevented from entering the growth atmosphere.
In Patent Document 1, source powder such as Ga and Na metal are placed into a crucible inside a glove box, and then the crucible is placed into a reaction vessel, from which impurities are removed and which is sealed. Then, the reaction vessel is removed from the glove box and is placed into a growth chamber. The nitrogen gas is introduced into the growth chamber to pressurize the growth chamber, whereby the reaction vessel is unsealed and the inner atmosphere is pressurized. This method allows prevention of the source material and the Na metal from being oxidized when the crucible is transferred from the glove box to the growth chamber.
Methods for growing gallium nitride crystals by a flux method, the method including adding carbon or a hydrocarbon to a starting source to prevent nucleation and inhibit formation of undesirable crystals, are also disclosed (Patent Documents 2 and 3). It is also disclosed that prevention of nucleation can be achieved by setting the growth temperature at a temperature higher than 850° C. (Non-Patent Document 1)
However, addition of carbon to the melt in the crucible as described in Patent Document 2 promotes the growth of nonpolar planes, which is responsible for inhibiting the growth of the polar c-plane.
Sodium metal, which is the starting material, is readily oxidizable. To place the crucible into the chamber without oxidation of the material, it is preferred to use an antioxidant vessel called an inner vessel for containing the crucible. Stainless steel has been found to be suitable as a material for the inner vessel from the viewpoint of durability. However, stainless steel has a maximum heat resistance temperature of about 870° C. at which the steel has a not significantly wide metastable region, and thus precise temperature control is required to suppress undesirable nucleation. Thus, three vessels are used, and supersaturation is controlled by precisely controlling the temperature in the chamber using a three-zone heater in order to grow crystals at a moderate rate without adding carbon or a hydrocarbon to the melt in the crucible and while preventing formation of undesirable crystals (Patent Document 4).
The applicants of the present invention have disclosed that, in a process of growing gallium nitride crystals by a flux method, a material such as zeolite, a molecular sieve, or alkali metal powder is placed into a space outside of a crucible and inside of a pressure vessel to scavenge trace amounts of oxygen in the pressure vessel, thereby preventing oxidation of the melt in the crucible (Patent Document 5).
The applicants of the present invention have also disclosed that, in a process of growing gallium nitride crystals by a flux method, an opening in the lid of a reaction vessel for containing a crucible is sealed with a sealant such as paraffin wax, polyethylene, or butyl rubber in a gas-tight manner, and nitrogen is introduced by melting the sealant by heating to open the opening in the lid (Patent Document 6, para. 0059).